Eating in the marine park is essentially eating on Ko Lipe — the other islands have either national-park canteens (basic but adequate) or no food at all. On Lipe the choice is wide for such a small place: dozens of restaurants line Walking Street and the back of each main beach. The standard offering is Thai (curries, stir-fries, fried rice), Indian-Thai fusion, and the universal banana pancake. A few places do real local fare worth seeking out.
The seafood barbecues along Walking Street and Pattaya Beach are the obvious choice — pick your fish from the ice display, agree on a price by weight, and it appears 20 minutes later grilled with chilli sauce and lime. Tuna, snapper, squid, and prawns are all good. The Urak Lawoi-run places tend to be cheaper and fresher than the more visible tourist restaurants; ask which ones are owned by the sea-gypsy families and the answer will usually involve a stand back from the main strip.
For breakfast, the small bakery cafes on Walking Street serve decent espresso and good baked goods. Coconut shake stalls are everywhere and worth the dollar they cost. Prices on Lipe are noticeably higher than on the mainland — everything has to come by boat — but a good meal is still cheap by global standards.